Class 11 Chemistry s-Block Elements NCERT Theory: Group 1 and Group 2 Properties, Reactions, Trends, Compounds, and Biological Importance

The s-Block Element – Class 11 NCERT Theory

Introduction:

The s-block elements include Group 1 (alkali metals) and Group 2 (alkaline earth metals).

These elements have their valence electron(s) in the s-orbital.

General electronic configuration:

Group 1: ns¹

Group 2: ns²

Group 1 Elements – Alkali Metals

Elements: Li, Na, K, Rb, Cs, Fr

Soft, silvery-white metals

Low melting and boiling points

Low density (Li, Na float on water)

Highly reactive, especially with water and air

Stored under kerosene

Physical Properties:

Large atomic and ionic size

Low ionization enthalpy

Good conductors of heat and electricity

Flame test: Li – crimson red, Na – golden yellow, K – violet

Chemical Properties:

Reactivity with air: Forms oxides, peroxides, or superoxides

Reactivity with water: Produces strong base (MOH) and H₂ gas

\ce{2Na + 2H2O -> 2NaOH + H2}

Important Compounds of Sodium:

NaCl – Common salt

NaOH – Strong base, caustic soda

Na2CO3.10H2O – Washing soda

NaHCO3 – Baking soda

Na2SO4.10H2O – Glauber’s salt

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Group 2 Elements – Alkaline Earth Metals

Elements: Be, Mg, Ca, Sr, Ba, Ra

Harder than alkali metals

Higher melting points and densities

Less reactive than Group 1 but reactivity increases down the group

Flame test: Ca – brick red, Sr – crimson, Ba – apple green

Physical Properties:

Smaller atomic and ionic sizes than Group 1

Higher ionization enthalpies

Form +2 oxidation state

Less electropositive than alkali metals

Chemical Properties:

Reaction with oxygen: Forms oxides (MO), some form peroxides (BaO₂)

Reaction with water:

Be does not react

Mg reacts slowly

Ca, Sr, Ba react readily forming hydroxides

\ce{Ca + 2H2O -> Ca(OH)2 + H2}

Reaction with acids: Liberates H₂ gas

Important Compounds of Calcium:

CaO (Quick lime) – Basic oxide, used in cement

Ca(OH)₂ (Slaked lime) – Used in whitewashing

CaCO₃ (Limestone) – Used in glass, cement, etc.

CaSO₄·½H₂O (Plaster of Paris) – Used in casts and building

CaCl₂ – Moisture absorbent

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Anomalous Behavior of Lithium and Beryllium:

Due to small size, high ionization energy, and high polarizing power

Lithium:

Forms covalent compounds

Li₂CO₃ decomposes easily

LiOH is a weak base

Shows diagonal relationship with Mg

Beryllium:

Amphoteric oxide and hydroxide

Forms covalent halides

Does not react with water

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Diagonal Relationship:

Li ↔ Mg and Be ↔ Al

Due to similar charge/radius ratio and electronegativity

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Trends in s-Block Elements:

Property Trend (Down the Group)

Atomic size Increases

Ionization enthalpy Decreases

Electropositivity Increases

Melting/boiling point Decreases (Group 1)

Reactivity Increases

Thermal stability Increases (carbonates)

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Biological Importance:

Na⁺ and K⁺: Maintain osmotic balance, nerve transmission

Mg²⁺: Present in chlorophyll, enzyme activator

Ca²⁺: Bones, teeth, blood clotting, muscle function

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